WASHINGTON—President Trump on Friday unleashed a half-dozen tweets lambasting the special counsel’s Russia investigation and vowing a counterattack, on a day in which two new court filings are expected to shed light on what investigators have learned from former associates of the president.

Mr. Trump tweeted that his lawyers had already written 87 pages of their response to a report that special counsel Robert Mueller is expected to file to the Justice Department upon the conclusion of his investigation. “Obviously cannot complete [the counter-report] until we see the final Witch Hunt Report,” Mr. Trump said.

The president also attacked the investigation, which is probing whether Trump associates colluded with Russia’s efforts to interfere in the 2016 U.S. election, as politically motivated. Mr. Mueller is investigating whether the president’s decision to fire then-FBI director James Comey in May 2017 while the FBI’s Russia investigation was under way amounted to obstruction of justice. Mr. Trump has denied any collusion or obstruction.

Mr. Trump on Friday also questioned the actions of his deputy attorney general, Rod Rosenstein, who took over control of the Russia investigation after former Attorney General Jeff Sessions recused himself in the spring of 2017. When Mr. Trump fired Mr. Comey, he cited a critical assessment of Mr. Comey’s job performance by Mr. Rosenstein.

Special counsel Robert Mueller is probing links between Trump adviser Roger Stone and WikiLeaks founder Julian Assange. WSJ’s Shelby Holliday looks at some of the possible connections between the two men. Photo: Getty Images.

Mr. Comey is set to testify on Friday in a closed joint session before the House Judiciary and Government Oversight committees, where he is expected to discuss his role in law-enforcement decisions during the 2016 election. The committees have said they would release a transcript of his testimony.

Friday will also see filings by Mr. Mueller’s office in the cases of two witnesses who have pleaded guilty: Paul Manafort, Mr. Trump’s former campaign chairman, and Michael Cohen, the president’s former longtime personal lawyer.

Mr. Mueller is due to submit a memo on Mr. Manafort, in which he is expected to lay out what alleged lies the former campaign chairman told investigators that prompted the office to effectively tear up his plea deal. Mr. Manafort in September pleaded guilty to conspiring against the U.S. and conspiring to obstruct justice, after being convicted on eight counts of tax and bank fraud in August.

Mr. Manafort’s alleged misstatements to investigators after his plea deal include comments about his personal business dealings and about his contacts with Konstantin Kilimnik, a former associate in Ukraine whom the FBI has assessed to have ties to Russian intelligence, according to people familiar with the matter and court filings.

Mr. Mueller is also expected to file a sentencing memorandum on behalf of Mr. Cohen, who pleaded guilty last week to lying to Congress about his efforts to build a Trump Tower in Moscow during the election, including about the extent to which Mr. Trump was informed about those efforts. The president’s lawyers have said they don’t dispute Mr. Cohen’s latest account.

In August, Mr. Cohen pleaded guilty to charges including violating campaign-finance laws by arranging hush-money payments to two women during the 2016 campaign after they alleged sexual affairs with Mr. Trump. Mr. Cohen told a federal judge that Mr. Trump ordered him to make the payments. Since then, Mr. Cohen has been cooperating with investigators for the Manhattan U.S. attorney’s office and Mr. Mueller’s office, among others, according to court filings.

Mr. Cohen’s lawyers last week asked a federal judge to spare their client jail time, stressing the “weighty and fraught” decision for Mr. Cohen to break with his longtime boss. Days later, Mr. Trump called for Mr. Cohen to serve a “full and complete” sentence, and suggested he receive a “long prison term.”

It is highly unusual for a president to weigh in directly on sentencing matters, particularly those related to a longtime former associate.